The authors aimed to investigate factors associated with smoking cessation among adolescents after tobacco intervention. They examined smokers (n = 127) from one birth cohort (n = 545) in the city of Kotka in Finland. These smokers were randomized in 3 intervention groups the dentist (n = 44) and the school nurse (n = 42 groups), and a control group (n = 39). After 2 months, the authors sent a follow-up questionnaire to the initial smokers to find out who had quit. The authors found that those whose best friend was a nonsmoker were more likely to stop smoking (relative risk RR 7.0 95% Cl 4.6-10.7). Moreover, the nicotine-dependent participants (measured according to the Fagerstrom Test for Nicotine Dependence) were less likely to stop (RR 0.1 95% Cl 0.08-0.11) compared to non-nicotine dependent participants. Last, of the diurnal types, the morning types found it easier to quit smoking than the evening types (RR 2.2 95% Cl 1.4-3.6). Thus, the authors concluded that the best friend's influence, nicotine dependence, and diurnal type could be taken more into account in individual counseling on smoking cessation. (Contains 1 figure and 1 table.)